igneous
Rounded dark mafic cobble, likely basalt
Basalt; primarily plagioclase feldspar, clinopyroxene, and commonly olivine or magnetite
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to weakly submetallic, with weathered rusty-brown patches; likely Mohs 5–6, massive and lacking visible cleavage. The rounded shape indicates water or abrasion transport rather than natural crystal form.
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Physical properties
Dark gray to black, fine-grained, dull to weakly submetallic, with weathered rusty-brown patches; likely Mohs 5–6, massive and lacking visible cleavage. The rounded shape indicates water or abrasion transport rather than natural crystal form.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its exact age cannot be determined from the photograph; the surface may be weathered or coated with iron oxides.
Uses & applications
Used extensively as crushed aggregate, road stone, riprap, railway ballast, and paving; attractive polished pieces can serve as decorative stone or lapidary material. This ordinary cobble generally has little commercial value.
Geological facts
Basalt is the most abundant volcanic rock and is compositionally equivalent to gabbro, which cools slowly underground. Brown coloration commonly reflects oxidation of iron-bearing minerals.
Field identification & locations
Identify it by its heavy, uniformly dark, fine-grained appearance, lack of fizz with dilute hydrochloric acid, and hardness sufficient to scratch glass in some specimens. Common along volcanic terrains and river or beach gravels worldwide; a hand lens may reveal tiny feldspar crystals or vesicles.
